Light emitting device comprising an organic led array on an ultra thin substrate

ABSTRACT

A light emitting device that comprises an organic LED array containing a plurality of light emitting pixels, the pixels each being located on a common electrically insulative transparent substrate, is characterized in that the transparent support is ultra thin, having a thickness less than the pitch of the pixels. The pixels in the LED array can be arranged in intersecting columns and rows, or they can comprise a line array. A process for forming a light emitting device on an ultra thin transparent substrate comprises the following steps: (a) releasably laminating to an ultra thin electrically insulating transparent substrate a relatively thick rigid temporary support; (b) forming the LED array on the ultra thin substrate; (c) forming on a relatively thick rigid permanent support an electrical conductor patterned in conformity with the first electrode element and the second electrode element on the ultra thin substrate; (d) forming an array of electrically conductive malleable bonding bumps on the patterned electrical conductor located on the permanent support; (e) aligning the first and second electrode elements in correct registration with the patterned electrical conductor; (f) contacting the bonding bumps on the electrical conductor with the first and second electrode elements; (g) forming a physical and electrical connection between the bonding bumps and the first and second electrode elements; and (h) delaminating the temporary support from the ultra thin transparent substrate. Additionally, a transparent and electrically insulative sealant material can be introduced into the space between the ultra thin substrate and the electrical conductor on the permanent support.

This is a Divisional of application Ser. No. 08/154,368 filed Nov. 18,1993, now U.S. Pat. No. 5,482,896.

FIELD OF THE INVENTION

This invention relates to a light emitting device comprising an organiclight-emitting diode (LED) array, and more particularly to a devicecomprising an organic LED array on an ultra thin transparent substrateand a process for forming the device.

BACKGROUND OF THE INVENTION

Organic LED arrays are typically employed as display devices, but theycan also serve as light sources for digital printing on photographicfilms and papers. For such applications, the print head would ordinarilyutilize a projection element for focusing the LED light source onto thephotosensitive element. This projection element can consist of a singlelens or an array of lenslets. However, because the radiant output of anorganic LED array is somewhat limited, it is important that anyprojection element so employed have very high light gatheringefficiency. Otherwise the quality of the digitally printed photographicimage would be significantly compromised.

PROBLEM TO BE SOLVED BY THE INVENTION

To avoid light gathering inefficiency, an organic LED array on an ultrathin transparent substrate fabricated in accordance with the process ofthe invention can be utilized in direct contact printing. This obviatesthe need for a projection element such as a lens and ensures the highestpossible light collection efficiency.

SUMMARY OF THE INVENTION

A light emitting device that comprises an organic LED array containing aplurality of light emitting pixels having a predetermined pitch; aplurality of light transmissive first electrode elements located on acommon electrically insulative transparent substrate, each firstelectrode element being laterally spaced and electrically insulated froman adjacent first electrode element; an organic electroluminescentmedium located on a supporting surface formed by the substrate and everyfirst electrode element; and a second electrode element located on theorganic electroluminescent medium, the pixels each including a firstelectrode, element and a second electrode element, is characterized inthat the electrically insulative transparent support is ultra thin,having a thickness less than the pitch of the pixels.

In one embodiment of the invention, the pixels of the device arearranged in a plurality of intersecting columns and rows; the pixels ineach column contain and are joined by a common first electrode element;and the pixels in each row contain and are joined by a common secondelectrode element, each second electrode: element being laterally spacedand electrically insulated from an adjacent second electrode element.

In another embodiment of the invention, the pixels of the device arearranged in a line array.

A process for forming a light emitting device on an ultra thintransparent substrate comprises the following steps: (a) releasablylaminating to an ultra thin electrically insulating transparentsubstrate a relatively thick rigid temporary support; (b) forming theLED array on the ultra thin substrate; (c) forming on a relatively thickrigid permanent support an electrical conductor patterned in conformitywith the first and second electrode elements located on the substrate.;(d) forming an array of electrically conductive malleable bonding bumpson the patterned electrical conductor located on the permanent support;(e) aligning the first and second electrode elements located on theultra thin substrate in correct registration with the patternedelectrical conductor located on the permanent support; (f) contactingthe bonding bumps on the electrical conductor with the first and secondelectrode elements; (g) forming a physical and electrical connectionbetween the bonding bumps and the first and second electrode elements:;and (h) delaminating the temporary support from the ultra thintransparent substrate. Additionally, a transparent and electricallyinsulative sealant material can be introduced into the space between theultra thin substrate and the electrical conductor on the permanentsupport.

ADVANTAGES OF THE INVENTION

Use of an ultra thin transparent substrate whose thickness dimension issmall relative to the lateral dimensions of a pixel of the LED arrayenables the light-emitting surface of the array to be brought into closeproximity to the photosensitive receptors of a photographic material,resulting in excellent optical coupling and minimum image distortion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view depicting a first electrodedeposited on an ultra thin substrate that has been laminated to atemporary support.

FIG. 2 is a schematic cross-sectional view of the structure of FIG. 1following patterning of the first electrode to form first electrodeelements and application of a dielectric layer.

FIG. 3 is a schematic cross-sectional view along line 3--3 of FIG. 2.

FIG. 4 is a schematic cross-sectional view of the structure of FIG. 3following patterning of the dielectric layer and application of anorganic electroluminescent (EL) medium, a second electrode element, anda capping layer.

FIG. 5 is a schematic cross-sectional view of the structure of FIG. 4inverted and with the patterned dielectric layer, EL medium, secondelectrode element, and capping layer schematically depicted as a singlecomposite layer to approximate more closely the relative thickness ofthe structural components.

FIG. 6 is a schematic cross-sectional view of a structure containingbonding bumps on a patterned electrical conductor located on a permanentsupport.

FIG. 7 is a schematic cross-sectional view depicting the contact inregistration of the structure of FIGS. 5 and 6.

FIG. 8 is a schematic cross-sectional view along line 8--8 of FIG. 7.

FIG. 9 is a schematic cross-sectional view along line 9--9 of FIG. 7.

FIG. 10 is a schematic cross-sectional view of a light emitting devicefabricated by the process of the invention.

Since dimensions of features of the device are frequently in thesub-micrometer range, the drawings are scaled for ease of visualizationrather than dimensional accuracy.

DETAILED DESCRIPTION OF THE INVENTION

A light emitting device fabricated according to the process of theinvention contains an organic LED array formed on an ultra thinsubstrate. This LED array can serve as a light source useful for directcontact printing on photographic, electrophotographic, and otherphotosensitive materials without the need for a projection lens system.Because the light source and the receptor medium can be as close asabout 10 μm apart, extremely high resolution printing is possible. Inaddition, there are potential savings in the manufacturing costs of aprinthead that requires no lens.

The fabrication process of the invention is illustrated by FIGS. 1-10.The figures depict an LED array containing a single row of pixels. Sucha line array is useful for digital printing applications. However, anLED array in which the pixels are arranged in a plurality of columns androws is also contemplated as an embodiment of the invention.

FIG. 1 depicts a releasably laminated ultra thin transparent substrate101 on a relatively thick rigid temporary support 102. For this purpose,an adhesive 103 can be employed. Suitable adhesives for this purposeinclude, for example, low temperature or hot melt adhesives.

The ultra thin substrate 101 is preferably glass, with a thickness of upto about 150 μm, preferably between about 25 μm and 50 μm. Glass havingthese characteristics is commercially available from Schott GlassCorporation and other vendors. An ultra thin plastic substrate can alsobe used, provided that it has dimensional and chemical stabilityproperties compatible with fabrication of the organic LED array.

The rigid temporary support 102 can be glass or other insulatingmaterials. Suitable thicknesses for this support are about 1 mm to 3 mm.

FIG. 1 also depicts the application of a light transmissive, preferablytransparent, first electrode 104 applied to the surface of the substrate101. Most commonly used for this purpose is indium tin oxide, applied byconventional methods such as sputtering. A suitable thickness for anindium tin oxide first electrode is from about 1000 Å to 4000 Å,preferably about 300 Å to 2500 Å.

FIG. 2 illustrates the first electrode patterned into first electrodeelements 204. Conventional photofabrication methods, such as photoresistpatterning followed by etching of unprotected indium tin oxide areaswith hydriodic or hydrochloric acid, followed in turn by removal of thephotoresist and rinsing, can be employed.

The first electrode elements 204 have a spatial resolution of about 100to 500 dots per inch (dpi), which is suitable for printing applications.Spatial resolution is the reciprocal of pixel pitch, which is defined asthe lateral distance between adjacent pixels. Thus, spatial resolutionsof 100 and 500 dpi correspond to pixel pitch dimensions of approximately250 μm and 50 μm, respectively. Conversely, a pitch of 150 μmcorresponds to resolution of about 170 dpi.

Also shown in FIG. 2 is a deposited dielectric layer 105, which can be apositive-working or negative-working photoresist material or suitableinorganic dielectric material such as SiO₂, Al₂ O₃, Si₃ N₄, and thelike. The dielectric layer 105 can comprise a single layer or multiplelayers having a total thickness of about 0.1 μm to 5 μm, preferablyabout 0.25 μm to 1 μm.

FIG. 3 is a section view along line 3--3 in FIG. 2; it shows only one ofthe first electrode elements 204.

The dielectric layer 105 can be patterned by conventionalphotofabrication and etching techniques to define the LED pixel areasand provide openings for electrical contacts, as represented by thepatterned dielectric layer 205 in the cross-sectional view depicted inFIG. 4. Also shown in FIG. 4 are the organic electroluminescent (EL)medium 206, the second electrode element 207, and the desiccant andcapping layer 208, applied sequentially to the patterned dielectriclayer 205 by vacuum deposition using shadow masks. Construction of thesecond electrode element 207 and application of the EL medium 206 andthe desiccant and capping layer 208 is carried out in such manner as toallow openings for the subsequent connection of the second electrodeelement to the electrical conductor, as shown in FIG. 6.

The organic EL medium 206 can comprise one or more layers and can bedeposited by conventional vapor deposition techniques. Its thickness,even in multilayer form, is considerably less than 1 μm and is typicallyless than 0.5 μm. Preferably, the organic EL medium consists of a holeinjection and transporting zone and an electron injecting andtransporting zone. More preferably, the hole injecting and transportingzone comprises a hole injecting layer adjacent to the anode of thedevice and a hole transporting layer, and the electron injecting andtransporting zone comprises an electron injecting layer adjacent to thecathode and an emitting layer in which hole and electron recombinationoccurs. In a preferred construction, first electrode elements form theanode and second electrode elements form the cathode of the device.

Suitable materials for the EL medium 206 for a device fabricated inaccordance with the present invention are disclosed in Scozzafava, EP349,265; Tang, U.S. Pat. No. 4,356,429; Van Slyke et al., U.S. Pat. No.4,539,507; Van Slyke et al., U.S. Pat. No. 4,720,432; Tang et al., U.S.Pat. No. 4,885,211; Tang et al., U.S. Pat. No. 4,769,292; Perry et al.,U.S. Pat. No. 4,950,950; Littman et al., U.S. Pat. No. 5,059,861; VanSlyke, U.S. Pat. No. 5,047,687; Scozzafava et al., U.S. Pat. No.5,073,446; Van Slyke et al., U.S. Pat. No. 5,059,862; Van Slyke et al.,U.S. Pat. No. 5,061,569; the disclosures of which are incorporatedherein by reference.

The second electrode element 207 is preferably constructed of acombination of a metal having a work function less that 4.0 eV and oneother metal, preferably a metal having a work function greater than 4.0eV. Suitable cathode materials are described in the previously mentionedU.S. Pat. Nos. 4,885,211, 5,059,862, and 5,073,446, the disclosures ofwhich are incorporated herein by reference. Mg:Al cathodes areespecially preferred.

The desiccant and capping layer 208 comprises an evaporated indium filmor an aluminum-organic matrix composite, as described in the previouslymentioned U.S. Pat. Nos. 5,073,446 and 5,059,862, the disclosures ofwhich are incorporated herein by reference.

FIG. 5 depicts the structure of FIG. 4 in an inverted orientation, withthe patterned dielectric layer 205, EL medium 206, second electrodeelement 207, and desiccant and capping layer 208 represented as a singlecomposite layer 209 to approximate more closely the relative thicknessof the structural components. The total thickness of the composite layer209 ranges from about 1 μm to 6 μm. Also shown in FIG. 5 is the activelight emitting portion 210 of a pixel; the area of the active portion istypically about 50-90 percent of the total area of the pixel.

In FIG. 6 is depicted a relatively thick (1-3 mm) rigid permanentsupport 110 on which is deposited an electrical conductor 111 that hasbeen patterned in conformity with the first and second electrodeelements 204 and 207, respectively: The electrical conductor 111 ispreferably formed of indium tin oxide or of an evaporated metal such asCr or Al.

Also depicted in FIG. 6 is an array of electrically conductive malleablebonding bumps 112 located on the patterned electrical conductor. Thebonding bumps are preferably formed of a soft metal alloy such as anindium alloy which can be deformed by pressure and/or melted by heating.

FIG. 7 and FIG. 8, which is a schematic cross-sectional view along line8--8 in FIG. 7, depicts the structures of FIGS. 5 and 6 brought intocontact so that the patterned electrical conductor 111 with itsprojecting bonding bumps 112 is in correct registration with the firstelectrode elements 204. Not depicted in FIGS. 7 or 8 are the secondelectrode element 207 and the bonding bumps in contact with it. Bondingbumps can be positioned to contact each end of all the first electrodeelements, as shown in FIGS. 7 and 8, or they can be arranged in aninterdigitated configuration, where only one end of each element, in analternating sequence, is in contact with a bonding bump.

FIG. 9, which is a schematic cross-sectional view along line 9--9 inFIG. 7, depicts bonding bumps 112 in registration and contact with thesecond electrode element 207. Not depicted in FIG. 9 is that portion ofthe patterned electrical conductor 111 beating the bonding bumps thatare in contact with the first electrode elements 204.

Mechanical pressure and/or localized heating can be employed to fuse thebonding bumps with the first and second electrode elements, resulting inthe establishment of a physical and electrical connection between theelectrical conductor and the first and second electrode elements.

FIG. 10 depicts a light emitting device 100 fabricated by the process ofthe invention, in which the final step is delamination of the rigidtemporary support 102 from the ultra thin substrate 101. Not shown inFIG. 10 are the second electrode element 207 or that portion of thepatterned electrical conductor in contact with it. Optionally, atransparent and electrically insulative sealant material 113 may beintroduced into the space between the ultra thin substrate 101 and theelectrical conductor 111 on the permanent support 110 to provide amoisture barrier and improve the mechanical strength of the structure.The sealant can be applied in a fluid state and subsequently caused orallowed to harden. Suitable sealant materials include, for example,epoxy resins, hot melt adhesives, and UV-curable adhesives.

The use of glass as the ultra thin substrate 101 provides anabrasive-resistant surface beneficial for contact printing. If desired,the glass surface can be further modified with a slipping layer such asa thin film of Teflon™.

Although the rigid temporary and permanent supports 102 and 110,respectively, have been represented in the drawings as flat, they canalso be curved. An appropriately curved surface may be advantageous forpaper transport in contact printing.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

    ______________________________________                                        PARTS LIST                                                                    ______________________________________                                        100          light emitting device                                            101          ultra thin substrate                                             102          rigid temporary support                                          103          adhesive                                                         104          first electrode                                                  105          dielectric layer                                                 110          rigid permanent support                                          111          electrical conductor                                             112          bonding bump                                                     113          sealant                                                          204          first electrode element                                          205          patterned dielectric layer                                       206          electroluminescent (EL) medium                                   207          second electrode element                                         208          desiccant and capping layer                                      209          composite layer                                                  210          light emitting portion of pixel                                  ______________________________________                                    

What is claimed is:
 1. A light emitting device that comprises an organicLED array containing a plurality of light emitting pixels having apredetermined pitch; a plurality of light transmissive first electrodeelements located on a common electrically insulative transparentsubstrate, each first electrode element being laterally spaced andelectrically insulated from an adjacent first electrode element; anorganic electroluminescent medium located on a supporting surface formedby the substrate and every first electrode element; and a secondelectrode element located on said organic electroluminescent medium,each said pixels including a first electrode element and a secondelectrode element,characterized in that said electrically insulativetransparent substrate is ultra thin, having a thickness less than thepitch of said pixels.
 2. A light emitting device according to claim 1wherein said pixels are arranged in a line array.
 3. A fight emittingdevice according to claim 1 including a permanent support and wherein apatterned electrical conductor is located on said permanent support. 4.A light emitting device according to claim 3 wherein said patternedelectrical conductor is physically and electrically connected to saidfirst electrode element and said second electrode element.
 5. A lightemitting device according to claim 3 wherein said patterned electricalconductor is comprised of indium tin oxide or an evaporated metal.
 6. Alight emitting device according to claim 5 wherein said metal is Cr orAl.
 7. A light emitting device according to claim 1 wherein saidelectrically insulative transparent substrate has a thickness of up toabout 150 μm.
 8. A light emitting device according to claim 7 whereinsaid transparent substrate has a thickness of about 25 μm to 50 μm.
 9. Alight emitting device according to claim 1 wherein said transparentsubstrate is glass.